FIG. 2A shows a typical arrangement of a conventional exposure apparatus.
In FIG. 2A, reference numeral 1 denotes a reticle; 2, a reticle stage for scanning the reticle 1; 3, a reticle stage guide of the reticle stage 2; 4, a projection system; 5, an alignment scope for measuring pattern positions; 6 and 7, focus detection systems (light projecting and light receiving units) of a focus measurement system for measuring the position of a wafer upper surface; 8, a wafer; 9, a chuck for holding the wafer; 10, a fine adjustment stage which can finely drive in the X, Y, Z, and θ (this represents rotation in a direction parallel to the X-Y plane (i.e., rotation about the Z axis) hereinafter) directions and in the tilt (this represents a tilt with respect to the X-Y plane hereinafter) direction; 11, a coarse adjustment stage which can drive the fine adjustment stage 10 in the X and Y directions; and 12, a wafer stage surface plate of the coarse adjustment stage 11.
In the conventional semiconductor exposure apparatus, the alignment scope 5 arranged adjacent to and at a distance BL from the projection system 4 measures the position of a pattern on the wafer 8. Then, the wafer 8 is fed by the coarse adjustment stage 11 to below the projection system 4. The reticle 1 and wafer 8 undergo a scan operation relative to the projection system 4 at a velocity of the magnification ratio of the projection system 4 such that a pattern on the reticle 1 is transferred onto a predetermined position on the wafer 8.
In the above-mentioned transfer, the focus measurement system measures the position of the upper surface of the wafer, and exposure operation is performed while the fine adjustment stage 10 performs sequential alignment in the focus direction such that the position of the upper surface of the wafer coincides with that of the image plane of the projection system 4.
The conventional semiconductor exposure apparatus has the following problems.
(1) Increase in Throughput
The conventional semiconductor exposure apparatus needs to measure the position of a pattern on the wafer 8 below the alignment scope 5 before exposure operation. This is one of the major factors which limits the throughput of the apparatus.
(2) Enhancement of Ease in Increasing Measurement Precision of Focus Detection System
The conventional semiconductor exposure apparatus needs to arrange the focus measurement system for the wafer below the projection system 4, as described above. For this reason, it is becoming difficult in terms of the mounting space, e.g., to implement a multichannel detection system or to improve a detection optical system to increase the measurement precision of the focus measurement system.
(3) Relaxation of Constraints on Design of Projection System
In designing the projection system 4, the above-mentioned focus measurement system is arranged below the projection system and, thus, a large back focal distance is necessary. This imposes considerable constraints on the design of the projection system 4. Along with a recent increase in the numerical aperture (NA) of the projection system 4, this problem has become serious. The problem will become serious in a mirror projection system of a future EUV exposure apparatus as well.
(4) Facilitation of Cleaning Below Projection System
Recently, contamination from a resist has been perceived as a problem. To prevent this, a jet of clean air is provided below the projection system 4. However, the focus measurement system described above is also arranged below the projection system 4, and thus it is difficult to form a complete laminar flow of clean air.
(5) Facilitation of Chuck Cleaning
As future exposure apparatuses, semiconductor exposure apparatuses using an F2 excimer laser or EUV light are being developed. In these semiconductor exposure apparatuses, the atmosphere for exposure light must be purged with nitrogen or must be evacuated to a vacuum. A semiconductor exposure apparatus used in such an environment needs to periodically extract a wafer chuck to the outer air side for cleaning. A conventional exposure apparatus, however, has no chuck transport function required for this operation.
To solve some problems of a conventional exposure apparatus, the following two methods are proposed. Their outlines will be described below.
(A) Place Two Coarse Adjustment Stages on Same Surface Plate
FIG. 2B shows the arrangement of improved method 1 in a conventional semiconductor exposure apparatus. Reference numeral 20 denotes an exposure wafer; 21, an exposure chuck; 22, an exposure fine adjustment stage; 23, an exposure coarse adjustment stage; 30, a measurement wafer; 31, a measurement chuck; 32, a measurement fine adjustment stage; and 33, a measurement coarse adjustment stage.
The semiconductor exposure apparatus according to improved method 1 has the two coarse adjustment stages, two fine adjustment stages, and the like. Exposure operation and measurement operations (alignment and focus measurements) can simultaneously and independently be performed for the wafers on the fine adjustment stages at exposure and measurement positions.
When predetermined processes end at the exposure and measurement positions, the fine adjustment stages are separated from the coarse adjustment stages and are interchanged. The wafer having undergone a measurement operation is moved to the exposure position for the exposure operation. Reference marks (not shown) are formed on the edges of the wafer chucks and are measured at the measurement and exposure positions. With this operation, measurement results (alignment and focus measurement results) at the measurement position are accurately reflected in exposure, and accurate alignment and focus are implemented at the exposure position.
Method 1 has advantages and disadvantages as follows.
(Advantages)
Exposure and measurement operations can be performed in parallel. If the time for the measurement operation is equal to or shorter than the time for the exposure operation, the measurement operation does not cause a decrease in throughput. Since enough time can be spared for the measurement operation, multipoint measurement or the like can be performed, and an increase in precision can be expected. Additionally, since a projection system and an alignment/focus measurement system are spaced apart from each other, constraints on the design of the projection system can be relaxed. Cleaning below the projection system is also facilitated.
(Disadvantages)
Independent operation of the two fine adjustment stages and two coarse adjustment stages increases the size of the entire stage and the complexity of a mechanism which interchanges the two fine adjustment stages. It is difficult to ensure a long term reliability and to perform replacement in a short time. Also, since the two stages operate independently of each other on one surface plate, their reaction forces make it difficult to keep the precision of scan synchronization between a reticle and wafers high at a high stage speed. Additionally, each stage basically has only a wafer transport function and requires considerable alterations to add the chuck unloading function described above.
Typical known examples of improved method 1 include PCT WO 98/28665, which is the corresponding publication of Japanese patent application number 2000-505958.
This known example discloses use of a counter mass to reduce effects of reaction forces generated by the independent operation of the two stages. Only one counter mass is used for the two stages, and it is difficult to completely remove the effects of the reaction forces.
As an example similar to the method shown in FIG. 2B, there is available the method discussed in Japanese Laid-Open No. 10-163098.
As in the above-mentioned known example, this known example has two stages capable of independent operation. The example proposes synchronization between the two stages in a specific operation for preventing any mutual interference between them and a reduction in size of the apparatus. This case may avoid any interference between the two stages and reduce the size of the apparatus. However, processing on one stage may be made to wait due to synchronization between the two stages, and a trade-off relationship is established between a reduction in size and an increase in throughput. Also, in this known example as well, the above-mentioned reaction force problem still remains unsolved.
As another example similar to the method shown in FIG. 2B, there is available the method discussed in Japanese Patent No. 3,045,947.
This known example is directed to a stepper. Similarly to the above-mentioned known example, the example has two stages capable of independent operation and proposes parallel stepwise operation of the two stages below exposure and measurement positions. This known example does not describe the structures of the two stages, and their details are unknown. Similarly to the method in FIG. 2B, it seems difficult to attain a stage performance as high as that of a single stage by the effects of the mutual reaction forces of the two stages.
(B) Two Completely Independent Stages
FIG. 2C shows the arrangement of improved method 2 in a conventional semiconductor exposure apparatus. Reference numeral 40 denotes an exposure wafer; 41, an exposure chuck; 42, an exposure fine adjustment stage; 43, an exposure coarse adjustment stage; 44, an exposure wafer stage surface plate; 50, a measurement wafer; 51, a measurement chuck; 52, a measurement fine adjustment stage; 53, a measurement coarse adjustment stage; and 54, a measurement wafer stage surface plate.
The semiconductor exposure apparatus using this method has two completely independent stages. After alignment and focus measurements on the measurement stage side, a wafer is loaded to the exposure stage side together with a chuck to perform exposure on the exposure stage side. As in improved method 1, reference marks (not shown) are formed on the edges of the chuck. The measurement and exposure stages measure the reference marks to accurately reflect alignment and focus measurement results on the measurement stage in exposure on the exposure stage. With this operation, accurate alignment and focus can be implemented.
Method 2 has advantages and disadvantages as follows.
(Advantages)
Method 2 basically has advantages similar to those of method 1. In this method, two stages including stage surface plates are completely independent, and they never exert reaction forces on each other. For this reason, even if the speed of each stage increases, the precision of scan synchronization between a reticle and a wafer can be kept high. Since method 2 basically adopts a wafer chuck transport method, it is relatively easy to implement the chuck unloading function.
(Disadvantages)
Since method 2 requires two sets of completely independent stages, the size of the apparatus increases. The lattices of the two sets of stages need to accurately coincide with each other. The two sets are slightly separated from each other, and it is more difficult than method 1 to cause the lattices to coincide with each other due to effects of the temperature, air pressure, gas molecule composition, humidity, and the like. Also, the method needs chuck transport. Since the two sets are separated from each other, it is necessary to hold a wafer so as to prevent the position of the wafer on a chuck from shifting from the chuck during the chuck transport.